Wireless communication method and wireless communication apparatus

ABSTRACT

Wireless communication is carried out between devices. A highest frequency band is selected. When the selected frequency band includes an unused channel in which no disturbing wave is present, a maximum transmission rate at which a received field strength value exceeds a threshold value is determined. When the selected frequency band does not include an unused channel or one in which no disturbing wave is present or there is no transmission rate associated with the selected frequency band at which the received field strength value exceeds the threshold value, the next highest frequency band is successively selected and the above is repeated. When the maximum transmission rate is successfully determined, communication is initiated using the unused channel of the selected frequency band at the maximum transmission rate as a communication channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a national stage application under 35 U.S.C.§371 of International Application No. PCT/JP03/05107, filed Apr. 22,2003, which claims priority from Japanese Application No. P2002-120518,filed Apr. 23, 2002, the disclosures of which are incorporated byreference herein.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a wireless communication method for usewith a wireless communication system constituted by a plurality ofwireless communication apparatuses and to wireless communicationapparatuses that composes the wireless communication system.

2. Background Art

A system constituted of a base apparatus and a display terminal has beendeveloped. The base apparatus functions as an information source or asan access point in which or to which a television broadcast receivingtuner is built or connected and that is connected to a telephone linethrough a modem as one type of a wireless LAN system that is structuredin a limited area of a residence, an office, or the like. The displayterminal executes functions for issuing a command to the base apparatusthrough a wireless communication with the base apparatus that includesreceiving a picture of the television from the base apparatus, receivinginformation from the Internet, displaying the picture and information ona display, and transmitting and receiving electronic mail through thebase apparatus.

As radio frequency bands that can be used for the wireless communicationsystem, the IEEE 802.11a standard has defined a 5.8 GHz band (in U.S. a5.2 GHz band, these bands are generally called 5 GHz band), whereas theIEEE 802.11b standard has defined a 2.4 GHz band.

When a wireless communication system deals with both the 5.2 GHz bandand the 2.4 GHz band, it can perform a communication over a radiochannel (radio frequency) properly selected as a communication channelfrom the 5.2 GHz band and 2.4 GHz band.

However, in a communicable area of the above-described wirelesscommunication system, another wireless communication system of the sametype as the present system or of a different type, such as Bluetooth (aregistered trademark), that uses the same frequency bands as the presentsystem might coexist.

In addition, if another system coexists with the present system, acommunication radio wave of the other system may become a disturbingwave that causes data streams to be broken, moving pictures streams tobe stopped, and images to be disturbed with respect to the datatransmission in the present system.

Also, besides those wireless communication systems that interfere withthe present system, there may be, for example, microwave ovens and soforth in the vicinity of the present system. When such a device radiatesa radio wave of the radio frequency band that the present system uses,the radio wave may act as a disturbing wave and adversely affect thepresent system.

Furthermore, when a large volume of data, such as picture data from thetelevision and moving picture data of the Internet, are transmitted, itwould be desirable to increase the data transmission rate.

However, in the 5.2 GHz band defined in the IEEE 802.11a standard, themaximum transmission rate can be increased up to at most 54 M bps (megabits/second). In contrast, in the 2.4 GHz band defined in the IEEE802.11b standard, the transmission rate can be increased up to at most11 Mbps.

If the transmission rate at which a large volume of data, such aspicture data and moving picture data, is transmitted is low, it might bedifficult to securely and smoothly transmit the data in a real timebasis.

SUMMARY OF THE INVENTION

Therefore, the present invention provides a wireless communicationsystem that deals with a plurality of communicable frequency bandshaving different transmission rates to allow a large volume of data tobe securely and smoothly transmitted in real time without a disturbancefrom another wireless communication system and so forth and withoutabnormalities such as the stopping of a moving picture and disturbanceof a still image.

A wireless communication method of the present invention is for use witha wireless communication system for performing a communication on aradio channel as a communication channel in a frequency band selectedfrom a plurality of communicable frequency bands having differenttransmission rates, the wireless communication method comprising thesteps of:

detecting radio channels that are not used in the system and that arefree of a disturbing wave transmitted from the outside of the systemfrom the frequency bands in decreasing order from relatively highertransmission rates;

detecting whether or not received field strengths at transmission ratesof the detected radio channels exceed a predetermined threshold value indecreasing order from the relatively higher transmission rates; and

starting a communication on a channel having a transmission rate atwhich the received field strength reaches or exceeds the predeterminedthreshold value in a manner that a communicable frequency band having arelatively higher transmission rate is prioritized and that an unusedchannel that is free of a disturbing wave and that has a relativelyhigher transmission rate in one of the frequency bands is prioritized asa communication channel in accordance with the results of the first andsecond detecting steps.

In the wireless communication method according to the present inventionof the above-described method, a frequency band communicable at a hightransmission rate is preferentially selected. A communication is startedon a communication channel that is free of a disturbing wavepreferentially at a high transmission rate. Therefore, the wirelesscommunication method according to the present invention is capable ofsecurely and smoothly transmitting a large volume of data on real timebasis without a disturbance of another wireless communication system andabnormalities of stop of a moving picture and a disturbance of an image.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects, features and advantages of the present inventionwill be further appreciated when considered with reference to thefollowing detailed description and accompanying drawings.

FIG. 1 is a schematic diagram showing an external structure of anexample of a display terminal used as a wireless communicationapparatus.

FIG. 2 is a schematic diagram showing an external structure of anexample of a base apparatus used as the wireless communicationapparatus.

FIG. 3 is a schematic diagram showing a raised state of the displayterminal.

FIG. 4 is a functional block diagram of the example of the structure ofthe base apparatus as the wireless communication apparatus shown in FIG.2.

FIG. 5 is a functional block diagram of the example of the structure ofthe display terminal as the wireless communication apparatus shown inFIG. 1.

FIG. 6 is a schematic diagram showing a channel structure of a 5.2 GHzband.

FIG. 7 is a schematic diagram showing a channel structure of a 2.4 GHzband.

FIG. 8 is a schematic diagram showing a transmission rate and modulationof the 5.2 GHz band.

FIG. 9 is a schematic diagram showing a transmission rate and modulationof the 2.4 GHz band.

FIG. 10 is a flow diagram showing a first part of an example of asetting process performed upon startup of communication.

FIG. 11 is a flow diagram showing a second part of the setting processshown in FIG. 10.

FIG. 12 is a flow diagram showing a first part of an example of a modesetting process for the 5.2 GHz band.

FIG. 13 is a flow diagram showing a second part of the mode settingprocess shown in FIG. 12.

FIG. 14 is a flow diagram showing an example of a mode setting processfor the 2.4 GHz band.

FIG. 15 is a flow diagram showing a first part of an example of achanging process performed during communication in the 5.2 GHz band.

FIG. 16 is a flow diagram showing a second part of the changing processshown in FIG. 15.

FIG. 17 is a flow diagram showing a first half part of an example of achanging process performed during communication in the 2.4 GHz band.

FIG. 18 is a flow diagram showing a second part of the changing processshown in FIG. 17.

FIG. 19 is a flow diagram showing an example of a mode changing processperformed during communication in the 5.2 GHz band when a transmissionrate is increased.

FIG. 20 is a flow diagram showing a first part of an example of a modechanging process performed during communication in the 2.4 GHz band whena transmission rate is decreased.

FIG. 21 is a flow diagram showing a second part of the mode changingprocess shown in FIG. 20.

DETAILED DESCRIPTION Best Mode for Carrying Out the Invention

Next, exemplifying the case in which the present invention is applied tothe wireless communication system, which is constituted by theabove-described base apparatus and display terminal, an embodiment ofthe present invention will be described.

External Structures of Display Terminal and Base Apparatus: FIG. 1 toFIG. 3:

FIG. 1 shows an external structure of an example of the displayterminal, and FIG. 2 shows an external structure of an example of thebase apparatus.

As shown in FIG. 1, an Liquid Crystal Display (LCD) 51 is disposed atthe front of a display terminal 50. A touch panel 53 is disposed on adisplay screen of the LCD 51. Speakers 55 are disposed at an upper leftposition and an upper right position of the LCD 51. Plain antennas 89 a,89 b for performing wireless communication with the base apparatus 10,which will be described later, are disposed at a lower left position anda lower right position of the LCD 51.

The antenna 89 a is used for a frequency band Ba (5.2 GHz band), whereasthe antenna 89 b is used for a frequency band Bb (a 2.4 GHz band). Theleft side antenna forms a semi-spherical surface radiation pattern inthe forward direction of the display terminal 50. The right side antennaforms a semi-spherical surface radiation pattern in the backwarddirection of the display terminal 50. One of the antennas is selected totransmit and receive a radio wave based on reception level informationof the left side antenna and the right side antenna. The combination ofthe left side antenna and the right side antenna forms an antenna thathas an all spherical surface radiation pattern. Regardless of therelation of positions of the display terminal 50 and the base apparatus10, wireless communication between the display terminal 50 and the baseapparatus 10 is securely performed.

Below the speaker 55 on the right side of the front of the displayterminal 50, an index button 57 a, a jump button 57 b, and channelbuttons 57 c, 57 d are disposed.

Pressing the index button 57 a causes an index screen, as shown in thedrawing, to be displayed on the LCD 51. By touching any item of the menuon the index screen with a touch pen or user's finger, the user canselect, for example, a channel of the television, operate an externaldevice that is connected to the base apparatus 10, access the Internet,create and transmit e-mail, and/or create and display an album.

Pressing the jump button 57 b, it allows an immediately precedingtelevision channel to be received. By pressing the channel button 57 c,the current mode displayed on the LCD 51 is switched in the order fromtelevision→external device→Internet→mail→album→television. By pressingthe channel button 57 d, the operation screen displayed on the LCD 51 isswitched in the reverse order.

An album is a picture or the like that is captured by a digital cameraand recorded on a the memory card 77 and which is then displayed on theLCD 51, processed on the LCD 51, and attached to e-mail created on theLCD 51. Alternatively, an album is a picture that is stored in thedisplay terminal 50 or in the memory card 77, a television imagecaptured as a still picture, a picture received by e-mail, a pictureobtained from the Internet, or the like.

On an upper surface of the display terminal 50, a groove portion 69 andso forth are formed. The groove portion 69 accommodates a touch pen 59.On the left side, a knob 91 and so forth are disposed. The knob 91adjusts the brightness of the LCD 51. On the right side, a slot 79 andso forth are formed. In the slot 79, the memory card 77 is attached. Atthe bottom, charging terminals 94, 96 are disposed.

At a the back surface of the display terminal 50, a U-shaped stand 99that allows the display terminal 50 to be raised is extensively andcontractively disposed. A battery accommodating portion (not shown) isdisposed at a portion surrounded by the stand 99. A battery isaccommodated in the battery accommodating portion.

As shown in FIG. 2, the base apparatus 10 is constituted of a frontportion 12 and a rear portion 14 that are integrally connected. At leftand right positions of the front portion 12, plain antennas 49 a, 49 bfor performing wireless communication with the display terminal 50 aredisposed.

Like the antennas 89 a, 89 b of the display terminal 50, the antenna 49a is used for the frequency band Ba (5.2 GHz band) whereas the antenna49 b is used for the frequency band Bb (2.4 GHz band). The left sideantenna forms a semi-spherical surface radiation pattern in the forwarddirection of the base apparatus 10. The right side antenna forms asemi-spherical surface radiation pattern in the backward direction ofthe base apparatus 10. In accordance with reception level information ofthe left side antenna and the right side antenna, one of the antennas isselected to transmit and receive a radio wave. The combination of theright side antenna and left side antenna forms an antenna that has ahalf-spherical surface radiation pattern. Regardless of the relation ofthe positions of the base apparatus 10 and the display terminal 50,wireless communication can be securely performed between the baseapparatus 10 and the display terminal 50.

The front portion 12 is slanted backwards and, in a lower center portionthereof, a supporting member 16 is disposed that causes the displayterminal 50 to be inclined against the base apparatus 10. Chargingterminals 24, 26 are disposed in the supporting member 16. In addition,at a lower portion on the rear surface of the rear portion 14, varioustypes of terminals, such as an antenna terminal and a line terminal thatwill be described later, are disposed.

With respect to the above-described base apparatus 10 and displayterminal 50, the user can place the base apparatus 10 at a fixedposition and carry the display terminal 50 to any place inside acommunicable area. The user can execute functions for receiving atelevision broadcast, accessing the Internet, and transmitting andreceiving electronic mail with the display terminal 50 that the user isholding at any place.

The user can operate the display terminal 50 by hand or, alternatively,with the stand 99 extended, as shown in FIG. 3, so that the displayterminal 50 can be raised on a proper surface at a properly inclinedangle.

In addition, the display terminal 50 may be inclined against the frontportion 12 of the base apparatus 10. In this case, the chargingterminals 94, 96 of the display terminal 50 are brought into contactwith the charging terminals 24, 26 of the base apparatus 10 and areconnected. As a result, the battery accommodated in the display terminal50 can be charged by the base apparatus 10.

Functional Blocks of Structures of Base Apparatus and Display Terminal:FIG. 4 and FIG. 5:

FIG. 4 shows the functional blocks of an example of a structure of thebase apparatus 10. A controlling portion 30 comprises a CPU 31. The CPU31 is connected to a bus 33. A program executed by the CPU 31, fixeddata, and so forth are written to a memory 35 in advance. The memory 35also functions as a work area and so forth of the CPU 31. The memory 35is connected to the bus 33.

An antenna 1 for receiving a television broadcast is connected to anantenna terminal 11. A television broadcast signal is received by theantenna 1 is channel-selected and demodulated by a tuner 21. Thechannel-selected and demodulated signal is then compressed and furtherconverted into video data and audio data. The video data and audio dataare then sent to the bus 33.

A telephone line 3 is connected to a line terminal 13. The line terminal13 is connected to the bus 33 through a modem 23.

In addition, an Ethernet (registered trademark) terminal 15, forconnecting an ADSL modem, a CATV modem, or the like, is connected to thebus 33 through an interface 25.

A DVD player, a hard disk recorder, a digital CS tuner, or the like isconnected as external device 7 to a terminal 17. Video data and audiodata from the external device 7 are sent to the bus 33 through aninterface 27.

In addition, an AV mouse 9 is connected to a terminal 19. The terminal19 is connected to the bus 33 through an interface 29. An infrared rayremote control signal is emitted by a light emitting portion of the AVmouse 9, in accordance with a command signal that is outputted from thecontrolling portion 30, and is received by a light detecting portiondisposed in the external device 7. As a result, using the infrared rayremote control signal, the external device 7 is operated.

Base Band Processors (BBP) 41 a, 41 b for the frequency bands Ba, Bb areconnected, respectively, to the bus 33. Transmitting and receivingportions 45 a, 45 b for the frequency bands Ba, Bb are connected to theBBPs 41 a, 41 b, respectively. The above-described antennas 49 a, 49 bare connected to the transmitting and receiving portions 45 a, 45 b,respectively.

Also, disturbing wave detecting portions 43 a, 43 b are connectedbetween the BBPs 41 a, 41 b and the bus 33, respectively. Received fieldstrength detecting portions 47 a, 47 b are connected between thetransmitting and receiving portions 45 a, 45 b and the bus 33,respectively. The disturbing wave detecting portions 43 a, 43 b detectwhether or not a disturbing wave exists on a radio channel selected fromthe frequency bands Ba, Bb, respectively, by a method that will bedescribed later. The received field strength detecting portions 47 a, 47b detect the received field strengths of the signals received by thetransmitting and receiving portions 45 a, 45 b in accordance withcontrol levels of an Automatic Gain Control (AGC) against the signalsreceived by the transmitting and receiving portions 45 a, 45 b,respectively.

A signal transmitted from the base apparatus 10 to the display terminal50 is processed for a baseband by the BBPs 41 a, 41 b and is thenmodulated by the transmitting and receiving portions 45 a, 45 b,respectively. The modulated signal is next converted into a signal of aradio channel selected from the frequency bands Ba, Bb. Thereafter, theradio channel signal is transmitted from the transmitting and receivingportions 45 a, 45 b to the display terminal 50 through the antennas 49a, 49 b, respectively.

In addition, a signal of a radio channel selected from the frequencybands Ba and Bb and transmitted from the display terminal 50 to baseapparatus 10 is received by the transmitting and receiving portions 45a, 45 b through the antennas 49 a, 49 b, respectively. The receivedsignal is frequency converted by the transmitting and receiving portions45 a, 45 b, respectively. Thereafter, the frequency converted signal isprocessed for a baseband by the BBPs 41 a, 41 b and then received by thebus 33.

FIG. 5 shows functional blocks of an example of a structure of thedisplay terminal 50. A controlling portion 70 comprises a CPU 71. TheCPU 71 is connected to a bus 73.

A program executed by the CPU 71, fixed data, and so forth are writtento a memory 75 in advance. The memory 75 also functions as a work areaand so forth of the CPU 71. The memory 75 is connected to the bus 73.

The LCD 51 is connected to the bus 73 through a display controllingportion 61. A speaker 55 is connected to the bus 73 through a D/Aconverter (DAC) 65 and an audio amplifying circuit 66. In addition, thetouch panel 53 is connected to the bus 73 through a coordinate detectingportion 63. Moreover, a key operation portion 57 including the indexbutton 57 a shown in FIG. 1, is connected to the bus 73 through aninterface 67.

When the memory card 77 is attached to the slot 79, shown in FIG. 1, thememory card 77 is connected to the bus 73.

In addition, BBPs 81 a, 81 b for the frequency bands Ba, Bb,respectively, are connected to the bus 73. Transmitting and receivingportions 85 a, 85 b for the frequency bands Ba, Bb are connected to theBBPs 81 a, 81 b, respectively. The above-described antennas 89 a, 89 bare connected to the transmitting and receiving portions 85 a, 85 b,respectively.

In addition, disturbing wave detecting portions 83 a, 83 b are connectedbetween the BBPs 81 a, 81 b and the bus 73, respectively. Received fieldstrength detecting portions 87 a, 87 b are connected between thetransmitting and receiving portions 85 a, 85 b and the bus 73,respectively. The disturbing wave detecting portions 83 a, 83 b detectwhether a disturbing wave is present in a radio channel selected fromthe frequency bands Ba, Bb, respectively, by a method that will bedescribed later. The received field strength detecting portions 87 a, 87b detect the received field strengths of signals received by thetransmitting and receiving portions 85 a, 85 b in accordance withcontrol levels of the AGC against signals received by the transmittingand receiving portions 85 a, 85 b, respectively.

A signal transmitted from the display terminal 50 to base apparatus 10is processed for a baseband by the BBPs 81 a, 81 b and is then modulatedby the transmitting and receiving portions 85 a, 85 b, respectively.Thereafter, the modulated signal is converted into a signal of a radiochannel selected from the frequency bands Ba, Bb. The signal of theradio channel is then transmitted from the transmitting and receivingportions 85 a, 85 b to the base apparatus 10 through the antennas 89 a,89 b, respectively.

In addition, a signal of a radio channel selected from the frequencybands Ba and Bb is transmitted from the base apparatus 10 to the displayterminal 50. The signal is received by the transmitting and receivingportions 85 a, 85 b through the antennas 89 a, 89 b, respectively. Thereceived signal is frequency converted and demodulated by thetransmitting and receiving portions 85 a, 85 b, respectively.Thereafter, the demodulated signal is processed for a baseband by theBBPs 81 a, 81 b and is then received by the bus 73.

Radio Frequency Bands, Radio Channels, and Transmission Rates: FIG. 6 toFIG. 9:

The above-described wireless communication system uses the 2.4 GHz bandand the 5.2 GHz band, as defined in the IEEE 802.11a standard and theIEEE 802.11b, standard as the frequency bands Ba and Bb, respectively.

It has been determined that when a plurality of radio channels are setin the 5.2 GHz band and 2.4 GHz at the same time and in the same area,as shown in FIG. 6 and FIG. 7, the frequency intervals between adjacentradio channels should be apart by 20 MHz or more and by 25 MHz or moreto prevent a signal from one radio channel from becoming a disturbingwave to a signal of the other radio channel, respectively.

Therefore, the number of radio channels that can be set at the same timein the 5.2 GHz band is a maximum of four channels C1, C2, C3, and C4,shown in FIG. 6. In the 2.4 GHz band is a maximum of three channels C5,C6, and C7 can be set, as shown in FIG. 7.

The transmission rates and modulation systems in the 5.2 GHz can be setin eight modes A1 to A8, shown in FIG. 8, and those in the 2.4 GHz bandin four modes B1 to B4, shown in FIG. 9. The terms “modes A1 to A8” and“modes B1 to B4” are not defined in the IEEE 802.11a and IEEE 802.11bstandards, but are defined in this specification for convenience.

Modulation systems BPSK, QPSK, QAM, and CCK, shown in FIGS. 8 and 9, areinitials for the following: BPSK: Binary Phase Shift Keying, QPSK:Quadrature Phase Shift Keying, QAM: Quadrature Amplitude Modulation, andCCK: Complementary Code Keying.

The modulation systems shown in FIG. 8 and FIG. 9 are multi-valuedigital modulation (primary modulation) systems for the BBPs 41 a and 41b of the base apparatus 10 and for the BBPs 81 a and 81 b of the displayterminal 50, respectively. Orthogonal Frequency Division Multiplexing(OFDM) is used as the radio frequency modulation for the frequency bandBa of the transmitting and receiving portion 45 a of the base apparatus10 and for the transmitting and receiving portion 85 a of the displayterminal 50. Direct Sequencing (DS) is used as the radio frequencymodulation for the frequency band Bb of the transmitting and receivingportion 45 b of the base apparatus 10 and of the transmitting andreceiving portion 85 b of the display terminal 50.

The transmission rate of the mode B4 in the 2.4 GHz band can be higherthan that of each of the modes A1 and A2 in the 5.2 GHz band as shown inthe FIG. 8 and FIG. 9. Generally, the transmission rate in the 5.2 GHzband can be higher than that in the 2.4 GHz band.

Setting Process Performed Upon Startup of Communication: FIG. 10 andFIG. 11:

In the state that the power of the base apparatus 10 has been turned onin the above-described wireless communication system, when the userturns on the power of the display terminal 50 and performs an operationfor receiving a television broadcast or performs an operation foraccessing the Internet using the display terminal 50, a connectionrequest and a command are transmitted from the display terminal 50 tothe base apparatus 10 as signals of a predetermined radio channel of apredetermined frequency band.

After the connection request and command have been received by the baseapparatus 10 and the operation for receiving a television broadcast orfor accessing the Internet has been performed, a communication with thedisplay terminal 50 is started. Picture and audio data of thetelevision, information of the Internet, and so forth are transmittedfrom the base apparatus 10 to the display terminal 50.

FIG. 10 and FIG. 11 show an example of a setting process for setting acommunication frequency band, a communication channel, and atransmission rate that the controlling portion 30 (CPU 31) of the baseapparatus 10 executes.

First, in step 101 of the setting process 100, the controlling portion30 determines whether an unused channel exists in the frequency band Ba(5.2 GHz band) in which a high transmission rate can be set.

While communication between the base apparatus 10 and a display terminalof the same type as that of the display terminal 50 is performed on aradio channel C1 to C4 as a communication channel in the frequency bandBa, the radio channel is not an unused channel. An unused channel is aradio channel that is not used as a communication channel by the presentsystem.

When the controlling portion 30 determines that an unused channel existsin the frequency band Ba in the step 101, the flow advances to step 102wherein the controlling portion 30 determines whether a disturbing waveexists on the unused channel based on the detected result of thedisturbing wave detecting portion 43 a for the frequency band Ba.

A disturbing wave is a communication radio wave transmitted from awireless communication system that is of the same type as or of adifferent type than the present system. Alternatively, the disturbingwave is a radio wave transmitted from a non-wireless communicationapparatus, such as a microwave oven.

When determining whether a signal received by the transmitting andreceiving portion 45 a is a disturbing wave, the disturbing wavedetecting portion 43 a and the controlling portion 30 detect whethertransmission destination address information is contained in a receivedsignal that has been processed in the BBP 41 a. When the transmissiondestination address information is contained therein, the controllingportion 30 determines whether the transmission destination addressinformation matches an apparatus address of the base apparatus 10.

When transmission destination address information is contained in thereceived signal and matches the apparatus address (identificationinformation that identifies an apparatus) of the base apparatus 10, thecontrolling portion 30 determines that the received signal is not adisturbing signal but is instead a signal transmitted from the displayterminal 50 to the base apparatus 10. When the received signal is aradio wave of other than a communication radio wave of another wirelesscommunication system and transmission destination address information isnot contained in the received signal or when the received signal is acommunication radio wave of another wireless communication system andtransmission destination address information contained in the receivedsignal, the controlling portion 30 determines that the received signalis a disturbing wave.

However, the system may be structured in a manner that, when thecontrolling portion 30 has determined that a received signal is adisturbing wave and the received field strength detecting portion 47 adetermines that the received field strength is so low that it can beignored, as shown in step 102, the controlling portion 30 determinesthat a disturbing wave does not exist on the unused channel.

When the controlling portion 30 has determined that a disturbing waveexists on the unused channel in step 102, the flow advances to step 103in which the controlling portion 30 determines whether another unusedchannel exists. When another unused channel exists, the flow returns tostep 102 where, in the same manner as described above, the controllingportion 30 determines whether a disturbing wave exists on the unusedchannel.

When the controlling portion 30 has determined that a disturbing wavedoes not exist on the unused channel in step 102, the flow advances tostep 104. After the controlling portion 30 has set the unused channel asa communication channel in step 104, the flow advances to processroutine 200. In process routine 200, the controlling portion 30 executesa mode setting process for the frequency band Ba.

In the mode setting process 200 for this frequency band Ba, as will bedescribed later with reference to FIG. 12 and FIG. 13, the controllingportion 30 detects received field strengths at transmission rates indecreasing order starting from higher transmission rates on thecommunication channel that has been set in step 104 of process routine100. The controlling portion 30 sets as a mode the highest transmissionrate at which the received field strength reaches or exceeds apredetermined threshold value.

After the controlling portion 30 has executed the mode setting process200, the flow advances to step 105 where the controlling portion 30determines whether communication should start in the frequency band Ba.When it is determined that the communication should start in thefrequency band Ba, the controlling portion 30 completes the settingprocess. The controlling portion 30 starts the communication in the mode(transmission rate) that was set in the process 200 on the communicationchannel that was set in step 104.

When the controlling portion 30 has determined that an unused channeldoes not exist in the frequency band Ba in step 101, has determined thatan unused channel free of a disturbing wave does not exist in thefrequency band Ba) in step 103, or has determined that an unused channelfree of a disturbing wave exists in the frequency band Ba but thereceived field strengths at all the transmission rates do not exceed thethreshold value) in step 105, the flow advances to step 111 wherein thecontrolling portion 30 determines whether an unused channel exists inthe frequency band Bb (2.4 GHz band).

When the controlling portion 30 has determined that an unused channelexists in the frequency band Bb, the flow advances from step 111 to step112, and the controlling portion 30 determines whether a disturbing waveexists on the unused channel in accordance with the detected result ofthe disturbing wave detecting portion 43 b for the frequency band Bb.

In this case, the controlling portion 30 determines whether the signalreceived by the transmitting and receiving portion 45 b is a disturbingwave and whether a disturbing wave exists on an unused channel in thesame manner as set out in step 102.

When the controlling portion 30 has determined that a disturbing waveexists on the unused channel in step 112, the flow advances to step 113wherein the controlling portion 30 determines whether another unusedchannel exists. When the controlling portion 30 has determined thatanother unused channel exists, the flow returns to step 112 where, inthe same manner as described above, the controlling portion 30determines whether a disturbing wave exists on the unused channel.

When the controlling portion 30 has determined that a disturbing wavedoes not exist on the unused channel in step 112, the flow advances tostep 114 wherein the controlling portion 30 sets the unused channel as acommunication channel. Thereafter, the flow advances to process routine300 in which the controlling portion 30 executes a mode setting processfor the frequency band Bb.

In the mode setting process 300 for the frequency band Bb, as will bedescribed later with reference to FIG. 14, the controlling portion 30detects a received field strength at the highest transmission rate onthe communication channel that was set in step 114 of the settingprocess 100. When the received field strength reaches or exceeds thethreshold value, the controlling portion 30 sets the transmission rateas a mode. When the received field strength does not reach the thresholdvalue, the controlling portion 30 sets the next highest transmissionrate as a mode.

When the controlling portion 30 has determined that an unused channeldoes not exist in the frequency band Bb in step 111 or has determinedthat an unused channel free of a disturbing wave does not exist in thefrequency band Bb, the flow advances to step 115. As step 115 shows, thecontrolling portion 30 sets a predetermined radio channel in apredetermined frequency band as a communication channel and sets apredetermined mode (transmission rate). For example, the controllingportion 30 sets a particular radio channel in the frequency band Ba (5.2GHz band) as a communication channel and sets mode A8 (transmissionrate: 54 Mbps) as a mode of the transmission rate. Thereafter, thecontrolling portion 30 completes the setting process performed uponstartup of communication and starts the communication.

Alternatively, in place of step 115 of process 100, the controllingportion 30 transmits a message indicating that data cannot betransmitted due to an improper communication environment between thebase apparatus 10 and the display terminal 50 and then causes themessage to be displayed on the LCD 51 of the display terminal 50 or tobe outputted as audio data from the speaker 55 to inform the user.

Mode Setting Process for Frequency Band Ba: FIG. 12 and FIG. 13:

FIGS. 12 and 13 show an example of a process routine 200 of a modesetting process for the frequency band Ba (5.2 GHz band).

When the controlling portion 30 starts communication in mode settingprocess e 200, the controlling portion 30 has already set an unusedchannel free of a disturbing wave in the frequency band Ba as acommunication channel in setting process 100. Thereafter, in step 211,the controlling portion 30 transmits a setup signal in mode A8(transmission rate: 54 Mbps), which has the highest transmission rate inthe frequency band Ba, from the base apparatus 10 to the displayterminal 50.

Thereafter, the flow advances to step 212 wherein the controllingportion 30 determines whether the received field strength at the timereaches or exceeds the threshold value.

As an example, the following method for detecting and determining thereceived field strength may be used. The transmitting and receivingportion 85 a of the display terminal 50 receives a signal transmittedfrom the base apparatus 10. The received field strength detectingportion 87 a of the display terminal 50 detects the received fieldstrength. The controlling portion 70 of the display terminal 50determines whether the received field strength reaches or exceeds thethreshold value and transmits the result from the display terminal 50 tothe base apparatus 10. The controlling portion 30 of the base apparatus10 then determines whether the received field strength reaches orexceeds the threshold value.

Alternatively, when the display terminal 50 has received a signal fromthe base apparatus 10, the display terminal 50 transmits an acknowledgesignal to the base apparatus 10 that notifies the base apparatus 10 thatthe display terminal 50 has received the signal. The transmitting andreceiving portion 45 a of the base apparatus 10 receives the acknowledgesignal. The received field strength detecting portion 47 a of the baseapparatus 10 then detects the received field strength. The controllingportion 30 of the base apparatus 10 next determines whether the receivedfield strength reaches or exceeds the threshold value.

When the controlling portion 30 has determined that the received fieldstrength in mode A8 reaches or exceeds the threshold value in step 212,the controlling portion 30 completes the mode setting process for thefrequency band Ba. When the controlling portion 30 starts thecommunication, the flow advances to step 105 of the setting process 100.In step 105, the controlling portion 30 determines that thecommunication should start in the frequency band Ba and then startscommunication in mode A8 on the communication channel that was set instep 104.

When the controlling portion 30 has determined that the received fieldstrength in mode A8 does not reach the threshold value in step 212, theflow advances to step 221. Here, the controlling portion 30 transmits asetup signal in mode A7 (transmission rate: 48 Mbps), which has thesecond highest transmission rate in the frequency band Ba, from the baseapparatus 10 to the display terminal 50. Thereafter, the flow advancesto step 222 wherein the controlling portion 30 determines whether thereceived field strength at the time reaches or exceeds the thresholdvalue in the same manner as described above.

Thereafter, when the controlling portion 30 has determined that thereceived field strength in mode A7 reaches or exceeds the thresholdvalue in step 222, the controlling portion 30 completes the mode settingprocess for the frequency band Ba. When the controlling portion 30starts communication, the flow advances to step 105 of the settingprocess 100 wherein the controlling portion 30 determines that thecommunication should start in the frequency band Ba and starts thecommunication in mode A7 on the communication channel that was set instep 104.

When the controlling portion 30 has determined that the received fieldstrength in mode A7 does not reach the threshold value in step 222, theflow advances to step 231. Here, the controlling portion 30 transmits asetup signal in mode A6 (transmission rate: 36 Mbps) from the baseapparatus 10 to the display terminal 50. Thereafter, the flow advancesto step 232 wherein the controlling portion 30 determines whether thereceived field strength at the time reaches or exceeds the thresholdvalue in the manner described above.

When the controlling portion 30 has determined that the received fieldstrength in mode A6 reaches or exceeds the threshold value in step 232,the controlling portion 30 completes the mode setting process for thefrequency band Ba. When the controlling portion 30 starts communicationin the setting process 100, the controlling portion 30 starts thecommunication in mode A6 in the same manner as described above.

When the controlling portion 30 has determined that the received fieldstrength in mode A6 does not reach the threshold value in step 232, theflow advances to step 241. Here, the controlling portion 30 transmits asetup signal in mode A5 (transmission rate: 24 Mbps) from the baseapparatus 10 to the display terminal 50. Thereafter, the flow advancesto step 242. The controlling portion 30 determines whether the receivedfield strength at the time reaches or exceeds the threshold value in theabove-described manner.

When the controlling portion 30 has determined that the received fieldstrength in mode A5 reaches or exceeds the threshold value in step 242,the controlling portion 30 completes the mode setting process for thefrequency band Ba. When the controlling portion 30 then startscommunication in the setting process 100, the controlling portion 30starts the communication in mode A5 in the same manner as describedabove.

When the controlling portion 30 has determined that the received fieldstrength in mode A5 does not reach the threshold value in step 242, theflow advances to step 251. Now, the controlling portion 30 transmits asetup signal in mode A4 (transmission rate: 18 Mbps) from the baseapparatus 10 to the display terminal 50. Thereafter, the flow advancesto step 252 in which the controlling portion 30 determines whether thereceived field strength at the time reaches or exceeds the thresholdvalue as described above.

When the controlling portion 30 has determined that the received fieldstrength in mode A4 reaches or exceeds the threshold value in step 252,the controlling portion 30 completes the mode setting process for thefrequency band Ba. When the controlling portion 30 next startscommunication in the setting process 100, the controlling portion 30starts the communication in mode A4 in the manner as described above.

When the controlling portion 30 has determined that the received fieldstrength in mode A4 does not reach the threshold value in step 252, theflow advances to step 261. Wherein, the controlling portion 30 transmitsa setup signal in mode A3 (transmission rate: 12 Mbps) from the baseapparatus 10 to the display terminal 50. Thereafter, the flow advancesto step 262 in which the controlling portion 30 determines whether thereceived field strength at the time reaches or exceeds the thresholdvalue in the same method as described above.

When the controlling portion 30 has determined that the received fieldstrength in mode A3 reaches or exceeds the threshold value in step 262,the controlling portion 30 completes the mode setting process for thefrequency band Ba. When the controlling portion 30 thereafter startscommunication in process routine 100, the controlling portion 30 startsthe communication in mode A3 in the manner described above.

When the controlling portion 30 has determined that the received fieldstrength in mode A3 does not reach the threshold value in step 262, theflow advances to step 271. Here, the controlling portion 30 transmits asetup signal in mode A2 (transmission rate: 9 Mbps) from the baseapparatus 10 to the display terminal 50. Thereafter, the flow advancesto step 272. The controlling portion 30 determines whether the receivedfield strength at the time reaches or exceeds the threshold value in thesame method as described above.

When the controlling portion 30 has determined that the received fieldstrength in mode A2 reaches or exceeds the threshold value in step 272,the controlling portion 30 completes the mode setting process for thefrequency band Ba. When the controlling portion 30 subsequently starts acommunication in the process 100, the controlling portion 30 starts thecommunication in mode A2 in the same manner as described above.

When the controlling portion 30 has determined that the received fieldstrength in mode A2 does not reach the threshold value in step 272, theflow advances to step 281. Where, the controlling portion 30 transmits asetup signal in mode A1 (transmission rate: 6 Mbps), which has thelowest transmission rate in the frequency band Ba, from the baseapparatus 10 to the display terminal 50. Thereafter, the flow advancesto step 282 in which the controlling portion 30 determines whether thereceived field strength at the time reaches or exceeds the thresholdvalue as described above.

When the controlling portion 30 has determined that the received fieldstrength in mode A1 reaches or exceeds the threshold value in step 282,the controlling portion 30 completes the mode setting process for thefrequency band Ba. When the controlling portion 30 next starts acommunication in the process 100, the controlling portion 30 starts thecommunication in mode A1 in the same manner as described above.

When the controlling portion 30 has determined that the received fieldstrength in mode A1 does not reach the threshold value in step 282, theflow advances to step 291. In step 291, the controlling portion 30 hasdetermined that no mode should be set in the frequency band Ba andcompletes the mode setting process for the frequency band Ba. When thecontrolling portion 30 starts communication, the flow advances to step105 of the setting process 100 wherein the controlling portion 30 hasdetermined that a communication should not start in the frequency bandBa. Here, as when the controlling portion 30 has determined that anunused channel does not exist in the frequency band Ba in step 101 or 30has determined that an unused channel free of a disturbing wave does notexist in the frequency band Ba in step 103, the flow advances to step111 as described above.

When the communication environment does not vary, the receivedsensitivity point, namely the received field strength of which the biterror rate of the received data does not reach a predetermined value,becomes higher, as the transmission rate is increased. Therefore, thethreshold values at the above-described steps 212, 222, 232, 242, 252,262, 272, and 282 are increased as the transmission rate increases.

Mode Setting Process for Frequency Band Bb: FIG. 14:

FIG. 14 shows an example of a mode setting process 300 for the frequencyband Bb (2.4 GHz band).

When the controlling portion 30 starts communication in the process 300,the flow first returns to step 114 of the process 100. In step 114, thecontrolling portion 30 sets an unused channel free of a disturbing wavein the frequency band Bb as a communication channel. Thereafter, theflow advances to step 311 where the controlling portion 30 transmits asetup signal in mode B4, which has the highest transmission rate in thefrequency band Bb (transmission rate: 11 Mbps), from the base apparatus10 to the display terminal 50.

Thereafter, the flow advances to step 312 in which the controllingportion 30 determines whether the received field strength at the timereaches or exceeds the threshold value.

As an example, the following method for detecting and determining thereceived field strength may be used. The transmitting and receivingportion 85 b of the display terminal 50 receives a signal transmittedfrom the base apparatus 10. The received field strength detectingportion 87 b of the display terminal 50 then detects the received fieldstrength. The controlling portion 70 of the display terminal 50 thendetermines whether the received field strength reaches or exceeds thethreshold value and transmits the result from the display terminal 50 tothe base apparatus 10. The controlling portion 30 of the base apparatus10 thereafter determines whether the received field strength reaches orexceeds the threshold value.

Alternatively, when the display terminal 50 has received a signaltransmitted from the base apparatus 10, the display terminal 50transmits an acknowledge signal to the base apparatus 10 that notifiesthe base apparatus that the display terminal 50 has received the signal.The transmitting and receiving portion 45 b of the base apparatus 10next receives the acknowledge signal. The received field strengthdetecting portion 47 b of the base apparatus 10 then detects thereceived field strength. The controlling portion 30 of the baseapparatus 10 subsequently determines whether the received field strengthreaches or exceeds the threshold value.

When the controlling portion 30 has determined that the received fieldstrength in mode B4 reaches or exceeds the threshold value in step 312,the controlling portion 30 completes the mode setting process for thefrequency band Bb and starts the communication in mode B4 on thecommunication channel that has been set in step 114 of the process 100.

When the controlling portion 30 has determined that the received fieldstrength in mode B4 does not reach the threshold value in step 312, theflow advances to step 313. Here, the controlling portion 30 sets mode B3(transmission rate: 5.5 Mbps), which has the second highest transmissionrate in the frequency band Bb, completes the mode setting process forthe frequency band Bb and then starts the communication in mode B3 onthe communication channel that has been set in step 114 of the settingprocess 100.

When the received field strength in mode B4 does not reach the thresholdvalue, the controlling portion 30 sets mode B3 without determiningwhether the received field strength in mode B3 reaches or exceeds thethreshold value because if the received field strength in mode B3 didnot exceed the threshold value and the controlling portion 30 were toset mode B2 (transmission rate: 2 Mbps) or mode B1 (transmission rate: 1Mbps), the resulting transmission rate would become too low.

Alternatively, the mode setting process 300 may be structured in thefollowing manner. When the controlling portion 30 has determined thatthe received field strength in mode 34 does not reach the thresholdvalue in step 312, the controlling portion 30 transmits a setup signalin mode B3 to the display terminal 50 and then determines whether thereceived field strength in mode B3 reaches or exceeds the thresholdvalue. When the received field strength reaches or exceeds the thresholdvalue, the controlling portion 30 sets mode B3. When the received fieldstrength does not reach the threshold value, the controlling portion 30determines that no mode is to be set in the frequency band Bb.

When the controlling portion 30 has determined that no mode is to be setin the frequency band Bb in the process 300, in the same manner that thecontrolling portion 30 determines that an unused channel exists in thefrequency band Bb in step 111 or determines that an unused channel freeof a disturbing wave does not exist in the frequency band Bb in step113, the flow advances to step 115. In step 115, the controlling portion30 sets a predetermined radio channel of a predetermined frequency bandas a communication channel, sets a predetermined mode (transmissionrate), and starts the communication.

Changing Process that is Performed During Communication: FIG. 15 to FIG.21:

Changing Process that is Performed During Normal Communication: FIG. 15to FIG. 18:

When the controlling portion 30 starts communication at a hightransmission rate in the frequency band Ba, if a disturbing wave ispresent in a communication channel, it is desirable to change thecommunication channel. Moreover, when the controlling portion 30 startscommunication at a low transmission rate in the frequency band Bb and anunused channel now exists in the frequency band Ba, it is desirable tochange the communication channel to the unused channel in the frequencyband Ba to increase the transmission rate.

Therefore, the above-described wireless communication system isstructured such that while the base apparatus 10 is communicating withthe display terminal 50, the controlling portion 30 of the baseapparatus 10 executes a changing process.

Changing Process Performed During Communication in Frequency Band Ba:FIG. 15 and FIG. 16:

FIG. 15 and FIG. 16 show an example of a changing process 120 performedduring communication in the frequency band Ba.

While the controlling portion 30 is communicating in the frequency bandBb, in step 129 of process routine 120, the controlling portion 30periodically determines whether a disturbing wave exists on acommunication radio channel in the frequency band Ba in accordance withthe detected result of the disturbing wave detecting portion 43 a.

In this case, as in the setting process 100 performed upon startup ofcommunication, when the controlling portion 30 determines whether asignal received by the transmitting and receiving portion 45 a is adisturbing wave, the above-described method for detecting/identifyingtransmission destination address information is used.

When the controlling portion 30 has determined that a disturbing waveexists on the communication radio channel in step 129, the controllingportion 30, in step 121, determines whether an unused channel exists inthe frequency band Ba. When it is determined that an unused channelexists, the flow advances to step 122 wherein the controlling portion 30determines whether a disturbing wave exists on the unused channel. Whensuch a disturbing wave is found to exist, the flow advances to step 123where the controlling portion 30 determines whether or not anotherunused channel exists. When it is determined that an unused channelexists, the flow returns to step 122 in which the controlling portion 30determines whether a disturbing wave exists on the unused channel.

When the controlling portion 30 has determined that a disturbing wavedoes not exist on the unused channel in step 122, the flow advances tostep 124 wherein the controlling portion 30 sets the unused channel as acommunication channel. Thereafter, the controlling portion 30 executesthe mode setting process 200 for the frequency band Ba. Thereafter, theflow advances to step 125 in which the controlling portion 30 determineswhether the communication should be continued in the frequency band Bain accordance with the result of the execution of the process 200. Whenthe controlling portion 30 has determined that the communication shouldbe continued in the frequency band Ba, the controlling portion 30restores the communicating state in the frequency band Ba.

When the controlling portion 30 has determined that an unused channeldoes not exist in the frequency band Ba in step 121, has determined thatan unused channel free of a disturbing wave does not exist in thefrequency band Ba in step 123, or has determined that an unused channelfree of a disturbing wave exists in the frequency band Ba but thereceived field strengths at all the transmission rates in the frequencyband Ba do not exceed the threshold value in step 125, the flow advancesto step 131. Namely, the controlling portion 30 determines whether anunused channel exists in the frequency band Bb.

When the controlling portion 30 has determined that an unused channelexists in the frequency band Bb, the flow advances from step 131 to step132 wherein the controlling portion 30 determines whether a disturbingwave exists on the unused channel. When it is determined that adisturbing wave exists, the flow advances to step 133 in which thecontrolling portion 30 determines whether another unused channel exists.When another unused channel exists, the flow returns to step 132.Namely, the controlling portion 30 determines whether or not adisturbing wave exists on the unused channel.

When the controlling portion 30 has determined that a disturbing wavedoes not exist on the unused channel in step 132, the flow advances tostep 134 wherein the controlling portion 30 sets the unused channel as acommunication channel. Thereafter, the controlling portion 30 executesthe mode setting process 300 for the frequency band Bb. The controllingportion 30 completes the changing process performed during communicationin the frequency band Ba and enters a communicating state in thefrequency band Bb.

When the controlling portion 30 has determined that an unused channeldoes not exist in the frequency band Bb in step 131 or has determinedthat an unused channel free of a disturbing wave does not exist in thefrequency band Bb in step 133, the flow advances to step 135. Namely,the controlling portion 30 continues the current communication for apredetermined time period and restores the communicating state in thefrequency band Ba.

Changing Process Performed During Communication in Frequency Band Bb:FIG. 17 and FIG. 18:

FIG. 17 and FIG. 18 show an example of a changing process performedduring communication in the frequency band Bb.

In changing process routine 140, while the controlling portion 30 iscommunicating, in step 147, the controlling portion 30 periodicallydetermines whether a disturbing wave exists on a communication radiochannel in accordance with the detected result of the disturbing wavedetecting portion 43 b.

In this case, as in the setting process routine 100 performed uponstartup of communication, when the controlling portion 30 determineswhether a signal received by the transmitting and receiving portion 45 bis a disturbing wave, the above-described method fordetecting/identifying transmission destination address information isused.

When the controlling portion 30 has determined that a disturbing waveexists on the communication radio channel in step 147, the flow directlyadvances from step 147 to step 141. When the controlling portion 30determines that the disturbing wave does not exist on the communicationradio channel, the flow advances from step 147 to step 149 wherein thecontrolling portion 30 continues the current communication for apredetermined time period. Thereafter, the flow advances to step 141.

In step 141, the controlling portion 30 determines whether an unusedchannel exists in the frequency band Ba, and when an unused channelexists, the flow advances to step 142. Namely, the controlling portion30 determines whether or not a disturbing wave exists on the unusedchannel and when a disturbing wave exists, the flow advances to step 143wherein the controlling portion 30 determines whether another unusedchannel exists. When it is determined that another unused channelexists, the flow returns to step 142 so that the controlling portion 30determines whether a disturbing wave exists on the unused channel.

When the controlling portion 30 has determined that a disturbing wavedoes not exist on the unused channel in step 142, the flow advances tostep 144 wherein the controlling portion 30 sets the unused channel as acommunication channel. Thereafter, the controlling portion 30 executes amode setting process 200 for the frequency band Ba. The flow thenadvances to step 145 in which the controlling portion 30 determineswhether the communication should be continued in the frequency band Ba.When it is determined that the communication should be continued in thefrequency band Ba, the controlling portion 30 completes the changingprocess performed during communication in the frequency band Ba.Thereafter, the controlling portion 30 enters a communicating state inthe frequency band Ba.

When the controlling portion 30 has determined that an unused channeldoes not exist in the frequency band Ba in step 141, has determined thatan unused channel free of a disturbing wave does not exist in thefrequency band Ba) in step 143, or has determined that an unused channelfree of a disturbing wave exists in the frequency band Ba but thereceived field strengths at all the transmission rates in the frequencyband Ba do not exceed the threshold value) in step 145, the flowadvances to step 151. Namely, the controlling portion 30 determineswhether an unused channel exists in the frequency band Bb.

When the controlling portion 30 has determined that an unused channelexists in the frequency band Bb, the flow advances from step 151 to step152 wherein the controlling portion 30 determines whether a disturbingwave exists on the unused channel. When it is determined that adisturbing wave exists, the flow advances to step 153 in which thecontrolling portion 30 determines whether another unused channel exists.When the controlling portion 30 has determined that another unusedchannel exists, the flow returns to step 152. Namely, the controllingportion 30 determines whether or not a disturbing wave exists on theunused channel.

When the controlling portion 30 has determined that a disturbing wavedoes not exist on the unused channel in step 152, the flow advances tostep 154 in which the controlling portion 30 sets the unused channel asa communication channel. Thereafter, the controlling portion 30 executesthe mode setting process 300 for the frequency band Bb and restores thecommunicating state in the frequency band Bb.

When the controlling portion 30 has determined that an unused channeldoes not exist in the frequency band Bb in step 151 or has determinedthat an unused channel free of a disturbing wave does not exist in thefrequency band Bb in step 153, the flow advances to step 155. Namely,the controlling portion 30 sets, for example, a predetermined radiochannel of a predetermined frequency band as a communication channel,sets a predetermined mode (transmission rate), and completes thechanging process performed during communication in the frequency bandBb.

Change of Transmission Rate: FIG. 19 to FIG. 21:

The Case in which Transmission Rate is Increased: FIG. 19:

When the controlling portion 30 starts a communication in the frequencyband Ba, even if the transmission rate cannot be increased because theelectric field is weak, if the environment of the electric field varies,the transmission rate may be increased. Thus, the system is structuredso that in that case the transmission rate can be increased.

FIG. 19 shows an example of a mode changing process 160 that thecontrolling portion 30 of the base apparatus 10 executes in such a case.

In the mode changing process 160, while the controlling portion 30 iscommunicating in the frequency band Ba, it periodically determineswhether a mode having a higher transmission rate than the current modeexists. When such a mode does not exist, namely, while the controllingportion 30 is communicating in mode A8 (transmission rate: 54 Mbps), theflow advances to step 162 wherein the controlling portion 30 continuesthe communication in the current mode (transmission rate).

When a mode that has a higher transmission rate than the current modeexists, namely, during communication in a mode lower than mode A7, theflow advances from step 161 to step 163 wherein the controlling portion30 changes the current mode to a mode having a higher transmission rate.Thereafter, in step 164, the controlling portion 30 determines whetherthe received field strength at the changed transmission rate reaches orexceeds the threshold value.

When the received field strength at the changed transmission rate doesnot reach the threshold value, the flow advances from step 164 to step165 so that the controlling portion 30 restores the preceding mode(transmission rate) from which the transmission rate was changed in step163 and continues the communication. When the received field strength atthe changed transmission rate reaches or exceeds the threshold value,the flow advances from step 164 to step 166 wherein the controllingportion 30 determines whether a mode having a higher transmission ratethan the current mode exists. When such a mode exists, the controllingportion 30 executes steps 163 and the steps after step 163. When a modehaving a higher transmission rate than the current mode does not exist,the flow advances to step 167. Namely, the controlling portion 30continues the communication in the mode (transmission rate) that waschanged in step 163.

For example, while the controlling portion 30 is communicating in modeA4, if the received field strength reaches or exceeds the thresholdvalue in mode A5, but not in mode A6, the controlling portion 30successively executes steps 161, 163, 164, 166, 163, 164, and 165. As aresult, the controlling portion 30 changes mode A4 to mode A4.

In contrast, while the controlling portion 30 is communicating in modeA7, if the received field strength reaches or exceeds the thresholdvalue in mode A8, the controlling portion 30 successively executes steps161, 163, 164, 166, and 167. As a result, the controlling portion 30changes mode A7 to mode A8.

The Case in which Transmission Rate is Decreased: FIG. 20 and FIG. 21:

In a good environment in which a radio wave is free of a disturbing wavewhile the controlling portion 30 is communicating at a high transmissionrate in the frequency band Ba, if the user having the display terminal50 goes away from the base apparatus 10, the electric field will varybecause the distance between the base apparatus 10 and the displayterminal 50 becomes large. In such case, the received field strengthbecomes lower than the received sensitivity point and as a result, thebit error rate of the received data becomes large and communicationcannot be securely performed. Thus, the system is structured for thecase where the transmission rate is decreased and the received fieldstrength becomes larger than the received sensitivity point.

FIGS. 20 and 21 show an example of a mode changing process 180 that thecontrolling portion 30 of the base apparatus 10 executes in such a case.

In the mode changing process 180, while the controlling portion 30 iscommunicating in the frequency band Ba, it periodically determineswhether the received field strength at the current transmission ratereaches or exceeds the received sensitivity point in step 181. When thereceived field strength does reach or exceed the received sensitivitypoint, the flow advances to step 182 wherein the controlling portion 30continues the communication in the current mode (transmission rate).

When the received field strength at that transmission rate does notreach the received sensitivity point, the flow advances from step 181 tostep 183. Namely, the controlling portion 30 determines whether a modehaving a lower transmission rate than the current mode exists. When sucha mode is determined to exist, the flow advances from step 183 to step184 wherein the controlling portion 30 changes the current mode to amode having a lower transmission rate by one level. Thereafter, the flowadvances to step 185 in which the controlling portion 30 determineswhether the received field strength at the changed transmission ratereaches or exceeds the received sensitivity point.

When the received field strength at the changed transmission ratereaches or exceeds the received sensitivity point, the flow advancesfrom step 185 to step 186. Namely, the controlling portion 30 continuesthe communication in the mode (transmission rate) changed in step 184.When the received field strength at the changed transmission rate doesnot reach the received sensitivity point, the flow advances from step185 to step 187 wherein the controlling portion 30 determines whether amode having a lower transmission rate than the current mode exists. Whensuch a mode exists, the controlling portion 30 repeats the stepsstarting from step 184.

For example, while the controlling portion 30 is communicating in modeA4, and if the received field strength does not reach the receivedsensitivity point but does reach or exceed the received sensitivitypoint in mode A3, the controlling portion 30 successively executes steps181, 183, 184, 185, and 186. As a result, the controlling portion 30changes from mode A4 to mode A3.

In contrast, when the controlling portion 30 has determined that a modehaving a lower transmission rate than the current mode does not exist instep 183, namely, while the controlling portion 30 is communicating inmode A1, if the received field strength does not reach the receivedsensitivity point, the flow advances to stop 191. Also, when thecontrolling portion 30 has determined that a mode having a lowertransmission rate than the current mode does not exist in step 187,namely even if the controlling portion 30 decreases the transmissionrate to mode A1 but the received field strength does not reach thereceived sensitivity point, the flow advances to step 191. In step 191,the controlling portion 30 determines whether or not an unused channelexists in the frequency band Bb.

When the controlling portion 30 has determined that an unused channelexists in the frequency band Bb, the flow advances from step 191 to step192 wherein the controlling portion 30 determines whether or not adisturbing wave exists on the unused channel. When it is determined thata disturbing wave exists, the flow advances to step 193. Namely, thecontrolling portion 30 determines whether another unused channel exists.When another unused channel exists, the flow returns to step 192 inwhich the controlling portion 30 determines whether a disturbing waveexists on the unused channel.

When the controlling portion 30 has determined that a disturbing wavedoes not exist on the unused channel in step 192, the flow advances tostep 194, namely, the controlling portion 30 sets the unused channel asa communication channel. Thereafter, the controlling portion 30 executesthe mode setting process 300 for the frequency band Bb. Thereafter, thecontrolling portion 30 enters a communicating state in the frequencyband Bb.

When the controlling portion 30 has determined that an unused channeldoes not exist in the frequency band Bb, in step 191, or has determinedthat an unused channel free of a disturbing wave does not exist in thefrequency band Bb, in step 193, the flow advances to step 195. Here, thecontrolling portion 30 continues the communication in mode A1 having thelowest transmission rate on the original communication channel in thefrequency band Ba, thus providing the highest possibility that thereceived field strength reaches or exceeds the received sensitivitypoint.

Other Embodiments

Frequency bands are currently defined in the IEEE standard and domesticstandard only at 5.2 GHz (5 GHz band) and at 2.4 GHz. However, it ispossible to use other frequency bands as the radio frequency bands ofthe invention. Other frequency bands may be defined in future. Thus thetwo frequency bands of the invention are not limited to 5.2 GHz (5 GHzband) and 2.4 GHz. In addition, the present invention can be applied tothe case in which three or more frequency bands are used.

In addition, the wireless communication apparatuses that compose thewireless communication system are not limited to the above-describedbase apparatus and display terminal.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, in a wirelesscommunication system that deals with a plurality of communicablefrequency bands having different transmission rates, a large volume ofdata can be securely and smoothly transmitted in real time without adisturbance from another wireless communication system and so forth, andabnormalities such as the stopping of a moving picture or a disturbanceof a still picture are avoided.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

The invention claimed is:
 1. In a first wireless communication device, amethod of carrying out wireless communication with a second wirelesscommunication device, said method comprising: (a) selecting a highestfrequency band from a plurality of frequency bands; (b) determiningwhether the selected frequency band includes an unused channel in whichno disturbing wave is present; (c) when the selected frequency bandincludes the unused channel in which no disturbing wave is present,determining, for a highest one of a plurality of transmission ratesassociated with the selected frequency band, whether a received fieldstrength value exceeds a threshold value, if the received field strengthvalue at the highest transmission rate exceeds the threshold value,initiating communication with the second wireless communication deviceusing the unused channel of the selected frequency band as acommunication channel at the highest transmission rate, if the receivedfield strength value at the highest transmission rate does not exceedthe threshold value, determining whether an immediately lower one of theplurality of transmission rates exceeds the threshold value, if thereceived field strength value at the immediately lower transmission rateexceeds the threshold value, initiating communication with the secondwireless communication device using the unused channel of the selectedfrequency band as a communication channel at the immediately lowertransmission rate, if the received field strength value at theimmediately lower transmission rate does not exceed the threshold value,(i) determining whether the received field strength value at a nextlower one of the plurality of transmission rates exceeds the thresholdvalue, (ii) if the received field strength value at the next lowertransmission rate exceeds the threshold value, initiating communicationwith the second wireless communication device using the unused channelof the selected frequency band as a communication channel at the nextlower transmission rate, (iii) if the received field strength value atthe next lower transmission rate does not exceed the threshold value,repeating steps (i) through (iii) until the received field strengthvalue at the next lower one of the plurality of transmission ratesexceeds the threshold value or until the next lower one of the pluralityof transmission rates is a lowest acceptable transmission rate, and (iv)if the next lower one of the plurality of transmission rates is thelowest acceptable transmission rate, initiating communication with thesecond wireless communication device using the unused channel of theselected frequency band as a communication channel at the next lowertransmission rate without determining whether the received fieldstrength value at the lowest acceptable transmission rate exceeds thethreshold value; and (d) when the selected frequency band (i) does notinclude an unused channel, or (ii) does not include an unused channel inwhich no disturbing wave is present, or (iii) includes the unusedchannel in which no disturbing wave is present but there is notransmission rate associated with the selected frequency band at whichthe received field strength value exceeds the threshold value, selectingthe next highest frequency band from the plurality of frequency bandsand repeating steps (b) through (d) using the next highest frequencyband as the selected frequency band.
 2. A method according to claim 1,wherein when none of the plurality of frequency bands includes an unusedchannel or when none of the plurality of frequency bands includes anunused channel in which no disturbing wave is present, said methodfurther comprises: setting a predetermined channel of a predeterminedfrequency band as the communication channel, setting a predeterminedtransmission rate for the communication channel, and then initiatingcommunication with the second wireless communication device using thecommunication channel at the predetermined transmission rate.
 3. Amethod according to claim 1, wherein when none of the plurality offrequency bands includes an unused channel or when none of the pluralityof frequency bands includes an unused channel in which no disturbingwave is present, said method further comprises: transmitting a messageto the second wireless communication device indicating thatcommunication cannot be carried out.
 4. A method according to claim 1,wherein said step of determining whether the selected frequency bandincludes an unused channel in which no disturbing wave is presentincludes: determining whether the selected frequency band includes afirst unused channel, when the selected frequency band includes thefirst unused channel, determining whether a disturbing wave is presentin the first unused channel, when no disturbing wave is present in thefirst unused channel, defining the first unused channel as the unusedchannel in which no disturbing wave is present, and when the disturbingwave is present in the first unused channel, (i) determining whether theselected frequency band includes another unused channel, (ii) when theselected frequency band includes the another unused channel, determiningwhether a disturbing wave is present in the another unused channel,(iii) when the disturbing wave is present in the another unused channel,repeating steps (i) through (iii) using yet another unused channel inplace of the another unused channel, and (iv) when no disturbing wave ispresent in the another unused channel, defining the another unusedchannel as the unused channel in which no disturbing wave is present. 5.In a first wireless communication device, a method of carrying outwireless communication with a second wireless communication device, saidmethod comprising: periodically determining, during communication withthe second wireless communication device using a particular channel of agiven one of a plurality of frequency bands as a communication channel,whether a disturbing wave is present in the communication channel; andwhen the disturbing wave is present in the communication channel, (a)determining whether the given one of the plurality of frequency bandsincludes an unused channel in which no disturbing wave is present, (b)when the given one of the plurality of frequency bands includes theunused channel in which no disturbing wave is present, determining amaximum transmission rate at which a received field strength valueexceeds a threshold value from a plurality of transmission ratesassociated with the given one of the plurality of frequency bands, (c)when the given one of the plurality of frequency bands (i) does notinclude an unused channel, or (ii) does not include an unused channel inwhich no disturbing wave is present, or (iii) includes the unusedchannel in which no disturbing wave is present but none of theassociated plurality of transmission rates provides a received fieldstrength value that exceeds the threshold value, substituting anotherone of the plurality of frequency bands for the given one of theplurality of frequency bands and then repeating steps (a) through (c),(d) when the maximum transmission rate is successfully determined instep (b), continuing communication with the second wirelesscommunication device using the unused channel as the communicationchannel at the determined maximum transmission rate; and (e) when noneof the plurality of frequency bands includes an unused channel or whennone of the plurality of frequency bands includes an unused channel inwhich no disturbing wave is present, continuing communication with thesecond wireless communication device for a predetermined time periodusing the given one of the plurality of frequency bands as thecommunication channel.
 6. A method according to claim 5, wherein saidstep of determining whether the given one of the plurality of frequencybands includes an unused channel in which no disturbing wave is presentincludes: determining whether the given one of the plurality offrequency bands includes a first unused channel, when the given one ofthe plurality of frequency bands includes the first unused channel,determining whether a disturbing wave is present in the first unusedchannel, when no disturbing wave is present in the first unused channel,defining the first unused channel as the unused channel in which nodisturbing wave is present, and when the disturbing wave is present inthe first unused channel, (i) determining whether the given one of theplurality of frequency bands includes another unused channel, (ii) whenthe given one of the plurality of frequency bands includes the anotherunused channel, determining whether a disturbing wave is present in theanother unused channel, (iii) when the disturbing wave is present in theanother unused channel, repeating steps (i) through (iii) using yetanother unused channel in place of the another unused channel, and (iv)when no disturbing wave is present in the another unused channel,defining the another unused channel as the unused channel in which nodisturbing wave is present.
 7. A method according to claim 5, whereinsaid step of determining a maximum transmission rate at which a receivedfield strength value exceeds the threshold value includes: determiningwhether a received field strength value at a highest one of theplurality of transmission rates transmission rate exceeds the thresholdvalue, when the received field strength value at the highest one of theplurality of transmission rates exceeds the threshold value, designatingthe highest one of the plurality of transmission rates as the maximumtransmission rate; and when the received field strength value at thehighest one of the plurality of transmission rates does not exceed thethreshold value, (i) determining whether the received field strengthvalue at a next lower one of the plurality of transmission rates exceedsthe threshold value, (ii) when the received field strength value at thenext lower one of the plurality of transmission rates exceeds thethreshold value, designating the next lower one of the plurality oftransmission rates as the maximum transmission rate, (iii) when thereceived field strength value at the next lower one of the plurality oftransmission rates does not exceed the threshold value, repeating steps(i) through (iii) until either the received field strength value for atleast one of the plurality of transmission rates exceeds the thresholdvalue or the next lower one of the plurality of transmission rates is alowest acceptable transmission rate, and (iv) when the next lower one ofthe plurality of transmission rates is the lowest acceptabletransmission rate, setting that transmission rate as the maximumtransmission rate without determining whether the received fieldstrength value at that transmission rate exceeds the threshold value. 8.In a first wireless communication device, a method of carrying outwireless communication with a second wireless communication device, saidmethod comprising: periodically determining, during communication withthe second wireless communication device at a particular one of aplurality of transmission rates associated with a given frequency band,whether the plurality of transmission rates includes at least onetransmission rate that is higher than the particular transmission rate;when the plurality of transmission rates includes the at least onetransmission rate higher than the particular transmission rate,determining whether a received field strength value at one of theplurality of transmission rates that is immediately higher than theparticular one of a plurality of transmission rates exceeds a thresholdvalue; when the plurality of transmission rates does not include the atleast one transmission rate higher than the particular transmission rateor when the received field strength value at the immediately higher oneof the plurality of transmission rates does not exceed the thresholdvalue, continuing the communication with the second wirelesscommunication device at the particular transmission rate; and when thereceived field strength value at the immediately higher one of theplurality of transmission rates exceeds the threshold value, (i)determining whether a next higher one of the plurality of transmissionrates exists, (ii) when the next higher one of the plurality oftransmission rates exists, determining whether the received fieldstrength value at the next higher one of the plurality of transmissionrates exceeds the threshold value, (iii) when the received fieldstrength value at the next higher one of the plurality of transmissionrates does not exceed the threshold value, continuing communication withthe second wireless communication device at an immediately lower one ofthe plurality of transmission rates, and (iv) when the received fieldstrength value at the next higher one of the plurality of transmissionrates exceeds the threshold value, repeating steps (i) through (iv)until the next higher one of the plurality of transmission rates is ahighest one of the plurality of transmission rates.
 9. In a firstwireless communication device, a method of carrying out wirelesscommunication with a second wireless communication device, said methodcomprising: periodically determining, during communication with thesecond wireless communication device at a particular one of a pluralityof transmission rates associated with a given frequency band, whether areceived field strength value at the particular one of the plurality oftransmission rates exceeds a threshold value; when the received fieldstrength value at the particular one of the plurality of transmissionrates exceeds the threshold value, continuing the communication with thesecond wireless communication device at the particular one of theplurality of transmission rates; when the received field strength valueat the particular one of the plurality of transmission rates does notexceed the threshold value, determining whether the received fieldstrength value at an immediately lower one of the plurality oftransmission rates exceeds the threshold value; when the received fieldstrength value at the immediately lower one of the plurality oftransmission rates exceeds the threshold value, continuing thecommunication with the second wireless communication device at theimmediately lower one of the plurality of transmission rates; and whenthe received field strength value at the immediately lower one of theplurality of transmission rates does not exceed the threshold value, (i)determining whether the received field strength value at a next lowerone of the plurality of transmission rates exceeds the threshold value,(ii) when the received field strength value at the next lower one of theplurality of transmission rates exceeds the threshold value, continuingthe communication with the second wireless communication device at thenext lower one of the plurality of transmission rates, (iii) when thereceived field strength value at the next lower one of the plurality oftransmission rates does not exceed the threshold value, repeating steps(i) through (iii) until either the received field strength value for atleast one of the plurality of transmission rates exceeds the thresholdvalue or until the next lower one of the plurality of transmission ratesis a lowest acceptable transmission rate, and (iv) when the next lowerone of the plurality of transmission rates is the lowest acceptabletransmission rate, setting that transmission rate as the maximumtransmission rate without determining whether the received fieldstrength value at that transmission rate exceeds the threshold value.10. A method according to claim 9, wherein when the received fieldstrength value at the least one of the plurality of transmission ratesdoes not exceed the threshold value, said method further comprises:selecting another frequency band, determining whether the anotherfrequency band includes an unused channel in which no disturbing wave ispresent, when the another frequency band includes the unused channel inwhich no disturbing wave is present, determining a maximum transmissionrate associated with the another frequency band at which the receivedfield strength value exceeds the threshold value, and continuingcommunication with the second wireless communication device using theunused channel as the communication channel at the determined maximumtransmission rate, and when the another frequency band does not includean unused channel or does not include an unused channel in which nodisturbing wave is present or when the another frequency band includesthe unused channel in which no disturbing wave is present but there isno transmission rate associated with the another frequency band at whichthe received field strength value exceeds the threshold value,continuing communication with the second wireless communication deviceusing the particular channel of the given frequency band as thecommunication channel at a lowest transmission rate associated with thegiven frequency band.
 11. A wireless communication apparatus forcarrying out wireless communication with another wireless communicationapparatus, said apparatus comprising: means for selecting a highestfrequency band from a plurality of frequency bands; means for (a)determining whether the selected frequency band includes an unusedchannel in which no disturbing wave is present, (b) when the selectedfrequency band includes the unused channel in which no disturbing waveis present, determining a maximum transmission rate at which a receivedfield strength value exceeds a threshold value from a plurality oftransmission rates associated with the selected frequency band, and (c)when the selected frequency band (i) does not include an unused channel,or (ii) does not include an unused channel in which no disturbing waveis present, or (iii) includes the unused channel in which no disturbingwave is present there is no transmission rate associated with theselected frequency band at which the received field strength valueexceeds the threshold value, selecting the next highest frequency bandfrom the plurality of frequency bands and repeating (a) through (c)using the next highest frequency band as the selected frequency band;and means for, when the maximum transmission rate is successfullydetermined, initiating communication with the another wirelesscommunication apparatus using the unused channel of the selectedfrequency band as a communication channel at the determined maximumtransmission rate; wherein said means for determining a maximumtransmission rate at which a received field strength value exceeds thethreshold value includes: means for determining whether a received fieldstrength value at a highest one of the plurality of transmission ratestransmission rate exceeds the threshold value, means for, when thereceived field strength value at the highest one of the plurality oftransmission rates exceeds the threshold value, designating the highestone of the plurality of transmission rates as the maximum transmissionrate, and means for, when the received field strength value at thehighest one of the plurality of transmission rates does not exceed thethreshold value, (i) determining whether the received field strengthvalue at a next lower one of the plurality of transmission rates exceedsthe threshold value, (ii) when the received field strength value at thenext lower one of the plurality of transmission rates exceeds thethreshold value, designating the next lower one of the plurality oftransmission rates as the maximum transmission rate, (iii) when thereceived field strength value at the next lower one of the plurality oftransmission rates does not exceed the threshold value, repeating (i)through (iii) until the received field strength value at one of theplurality of transmission rates exceeds the threshold value or the nextlower one of the plurality of transmission rates is a lowest acceptabletransmission rate, and (iv) when the next lower one of the plurality oftransmission rates is the lowest acceptable transmission rate, settingthat transmission rate as the maximum transmission rate withoutdetermining whether the received field strength value at thattransmission rate exceeds the threshold value.
 12. An apparatusaccording to claim 11, further comprising: means for, when none of theplurality of frequency bands includes an unused channel or when none ofthe plurality of frequency bands includes an unused channel in which nodisturbing wave is present, setting a predetermined channel of apredetermined frequency band as the communication channel, setting apredetermined transmission rate for the communication channel, and theninitiating communication with the another wireless communicationapparatus using the communication channel at the predeterminedtransmission rate.
 13. An apparatus according to claim 11, furthercomprising: means for, when none of the plurality of frequency bandsincludes an unused channel or when none of the plurality of frequencybands includes an unused channel in which no disturbing wave is present,transmitting a message to the another wireless communication apparatusindicating that communication cannot be carried out.
 14. An apparatusaccording to claim 11, wherein said means for determining whether theselected frequency band includes an unused channel in which nodisturbing wave is present includes: means for determining whether theselected frequency band includes a first unused channel, means for, whenthe selected frequency band includes the first unused channel,determining whether a disturbing wave is present in the first unusedchannel, means for, when no disturbing wave is present in the firstunused channel, defining the first unused channel as the unused channelin which no disturbing wave is present, and means for, when thedisturbing wave is present in the first unused channel, (i) determiningwhether the selected frequency band includes another unused channel,(ii) when the selected frequency band includes the another unusedchannel, determining whether a disturbing wave is present in the anotherunused channel, (iii) when the disturbing wave is present in the anotherunused channel, repeating (i) through (iii) using yet another unusedchannel in place of the another unused channel, and (iv) when nodisturbing wave is present in the another unused channel, defining theanother unused channel as the unused channel in which no disturbing waveis present.
 15. A wireless communication apparatus for carrying outwireless communication with another wireless communication apparatus,said apparatus comprising: means for periodically determining, duringcommunication with the another wireless communication apparatus using aparticular channel of a given one of a plurality of frequency bands as acommunication channel, whether a disturbing wave is present in thecommunication channel; and means for, when the disturbing wave ispresent in the communication channel, (a) determining whether the givenone of the plurality of frequency bands includes an unused channel inwhich no disturbing wave is present, (b) when the given one of theplurality of frequency bands includes the unused channel in which nodisturbing wave is present, determining a maximum transmission rate atwhich a received field strength value exceeds a threshold value from aplurality of transmission rates associated with the given one of theplurality of frequency bands, (c) when the given one of the plurality offrequency bands (i) does not include an unused channel, or (ii) does notinclude an unused channel in which no disturbing wave is present, or(iii) includes the unused channel in which no disturbing wave is presentbut none of the associated plurality of transmission rates provides areceived field strength value that exceeds the threshold value,substituting another one of the plurality of frequency bands for thegiven one of the plurality of frequency bands and then repeating (a)through (c), (d) when the maximum transmission rate is successfullydetermined, continuing communication with the another wirelesscommunication apparatus using the unused channel as the communicationchannel at the determined maximum transmission rate, and (e) when noneof the plurality of frequency bands includes an unused channel or whennone of the plurality of frequency bands includes an unused channel inwhich no disturbing wave is present, continuing communication with theanother wireless communication apparatus for a predetermined time periodusing the given one of the plurality of frequency bands as thecommunication channel.
 16. An apparatus according to claim 15, whereinsaid means for determining whether the given one of the plurality offrequency bands includes an unused channel in which no disturbing waveis present includes: means for determining whether the given one of theplurality of frequency bands includes a first unused channel, means for,when the given one of the plurality of frequency bands includes thefirst unused channel, determining whether a disturbing wave is presentin the first unused channel, means for, when no disturbing wave ispresent in the first unused channel, defining the first unused channelas the unused channel in which no disturbing wave is present, and meansfor, when the disturbing wave is present in the first unused channel,(i) determining whether the given one of the plurality of frequencybands includes another unused channel, (ii) when the given one of theplurality of frequency bands includes the another unused channel,determining whether a disturbing wave is present in the another unusedchannel, (iii) when the disturbing wave is present in the another unusedchannel, repeating (i) through (iii) using yet another unused channel inplace of the another unused channel, and (iv) when no disturbing wave ispresent in the another unused channel, defining the another unusedchannel as the unused channel in which no disturbing wave is present.17. An apparatus according to claim 15, wherein said means fordetermining a maximum transmission rate at which a received fieldstrength value exceeds the threshold value includes: means fordetermining whether a received field strength value at a highest one ofthe plurality of transmission rates transmission rate exceeds thethreshold value, means for, when the received field strength value atthe highest one of the plurality of transmission rates exceeds thethreshold value, designating the highest one of the plurality oftransmission rates as the maximum transmission rate; and means for, whenthe received field strength value at the highest one of the plurality oftransmission rates does not exceed the threshold value, (i) determiningwhether the received field strength value at a next lower one of theplurality of transmission rates exceeds the threshold value, (ii) whenthe received field strength value at the next lower one of the pluralityof transmission rates exceeds the threshold value, designating the nextlower one of the plurality of transmission rates as the maximumtransmission rate, (iii) when the received field strength value at thenext lower one of the plurality of transmission rates does not exceedthe threshold value, repeating (i) through (iii) until either thereceived field strength value for at least one of the plurality oftransmission rates exceeds the threshold value exceeds the thresholdvalue or the next lower one of the plurality of transmission rates is alowest acceptable transmission rate, and (iv) when the next lower one ofthe plurality of transmission rates is the lowest acceptabletransmission rate, setting that transmission rate as the maximumtransmission rate without determining whether the received fieldstrength value at that transmission rate exceeds the threshold value.18. A wireless communication apparatus for carrying out wirelesscommunication with another wireless communication apparatus, saidapparatus comprising: means for periodically determining, duringcommunication with the another wireless communication apparatus at aparticular one of a plurality of transmission rates associated with agiven frequency band, whether the plurality of transmission ratesincludes at least one transmission rate that is higher than theparticular transmission rate; means for, when the plurality oftransmission rates includes the at least one transmission rate higherthan the particular transmission rate, determining whether a receivedfield strength value at one of the plurality of transmission rates thatis immediately higher than the particular one of a plurality oftransmission rates exceeds a threshold value; means for, when theplurality of transmission rates does not include the at least onetransmission rate higher than the particular transmission rate or whenthe received field strength value at the immediately higher one of theplurality of transmission rates does not exceed the threshold value,continuing the communication with the another wireless communicationapparatus at the particular transmission rate; and means for, when thereceived field strength value at the immediately higher one of theplurality of transmission rates exceeds the threshold value, (i)determining whether a next higher one of the plurality of transmissionrates exists, (ii) when the received field strength value at the nexthigher one of the plurality of transmission rates exists, determiningwhether the received field strength value at the next higher one of theplurality of transmission rates exceeds the threshold value, (iii) whenthe received field strength value at the next higher one of theplurality of transmission rates does not exceed the threshold value,continuing communication with the another wireless communicationapparatus at an immediately lower one of the plurality of transmissionrates, and (iv) when the received field strength value at the nexthigher one of the plurality of transmission rates exceeds the thresholdvalue, repeating (i) through (iv) until the next higher one of theplurality of transmission rates is a highest one of the plurality oftransmission rates.
 19. A wireless communication apparatus for carryingout wireless communication with another wireless communicationapparatus, said apparatus comprising: means for periodicallydetermining, during communication with the another wirelesscommunication apparatus at a particular one of a plurality oftransmission rates associated with a given frequency band, whether areceived field strength value at the particular one of the plurality oftransmission rates exceeds a threshold value; means for, when thereceived field strength value at the particular one of the plurality oftransmission rates exceeds the threshold value, continuing thecommunication with the another wireless communication apparatus at theparticular one of the plurality of transmission rates; means for, whenthe received field strength value at the particular one of the pluralityof transmission rates does not exceed the threshold value, determiningwhether the received field strength value at an immediately lower one ofthe plurality of transmission rates exceeds the threshold value; meansfor, when the received field strength value at the immediately lower oneof the plurality of transmission rates exceeds the threshold value,continuing the communication with the another wireless communicationapparatus at the immediately lower one of the plurality of transmissionrates; and means for, when the received field strength value at theimmediately lower one of the plurality of transmission rates does notexceed the threshold value, (i) determining whether the received fieldstrength value at a next lower one of the plurality of transmissionrates exceeds the threshold value, (ii) when the received field strengthvalue at the next lower one of the plurality of transmission ratesexceeds the threshold value, continuing the communication with theanother wireless communication apparatus at the next lower one of theplurality of transmission rates, (iii) when the received field strengthvalue at the next lower one of the plurality of transmission rates doesnot exceed the threshold value, repeating (i) through (iii) until eitherthe received field strength value for at least one of the plurality oftransmission rates exceeds the threshold value or until the next lowerone of the plurality of transmission rates is a lowest acceptabletransmission rate, and (iv) when the next lower one of the plurality oftransmission rates is the lowest acceptable transmission rate, settingthat transmission rate as the maximum transmission rate withoutdetermining whether the received field strength value at thattransmission rate exceeds the threshold value.
 20. An apparatusaccording to claim 19, further comprising: means for, when the receivedfield strength value at the least one of the plurality of transmissionrates does not exceed the threshold value, selecting another frequencyband, determining whether the another frequency band includes an unusedchannel in which no disturbing wave is present, when the anotherfrequency band includes the unused channel in which no disturbing waveis present, (i) determining a maximum transmission rate associated withthe another frequency band at which the received field strength valueexceeds the threshold value, and (ii) continuing communication with theanother wireless communication apparatus using the unused channel as thecommunication channel at the determined maximum transmission rate, andwhen the another frequency band does not include an unused channel ordoes not include an unused channel in which no disturbing wave ispresent or when the another frequency band includes the unused channelin which no disturbing wave is present but there is no transmission rateassociated with the another frequency band at which the received fieldstrength value exceeds the threshold value, continuing communicationwith the another wireless communication apparatus using the particularchannel of the given frequency band as the communication channel at alowest transmission rate associated with the given frequency band.